Measuring cell for the infrared analysis of fluids, measuring system having such a measuring  cell, and method for producing such a measuring cell

ABSTRACT

The invention relates to a measuring cell ( 1 ) for the infrared analysis of fluids, in particular a measuring cell ( 1 ) having a permissible operating pressure of more than 20 bar and preferably more than 50 bar, having a flow channel ( 10 ) for the fluid that is formed between a first and a second element ( 2, 4 ), which are each transparent to infrared radiation at least in some sections, wherein the infrared radiation can be radiated into the flow channel ( 10 ) by means of the first clement ( 2 ) and can exit the flow channel ( 10 ) by means of the second element ( 4 ), and wherein the two elements ( 2, 4 ) are connected to each other in a fluid-tight and mechanically high-strength manner by means of a connecting layer ( 6 ) that is arranged therebetween and that is made of a glass-containing material, in particular a sintered glass-ceramic material; the invention further relates to a measuring system ( 8 ) having such a measuring cell ( 1 ) and to a method for producing such a measuring cell ( 1 ).

The invention relates to a measuring cell for the infrared analysis offluids, a measuring system having such a measuring cell, and a methodfor producing such a measuring cell.

A measuring cell of this type can be used, for example, for the analysisof oils which are used in technical systems for the transmission ofpressures, for lubrication, and/or for cooling. In operation, the oil issubjected to aging and/or fouling, and for the operational reliabilityof the system, it is critical to be able to check the quality state ofthe oil in near real time. For this purpose, the wavelength-dependenttransmission of the oil can be measured, or the absorption bands can bemeasured especially in the infrared range, and conclusions can be drawntherefrom regarding the quality of the oil.

Reflection spectrometers with these measuring cells are known, forexample, from DE 103 21 472 A1, DE 197 31 241 C2, or EP 0 488 947 A1.Transmission spectrometers are known, for example, from DE 10 2004 008685 A1 and GB 2 341 925 A.

DE 41 37 060 C2 shows a microcell for infrared spectroscopy.

US 2002/0063330 A1 shows a heat sink and a method for producing thisheat sink.

DE 102 44 786 A1 and AT 500 075 B1 show a method for connecting wafers.DE 103 29 866 A1 shows the use of wafer bonding for a piezoelectricsubstrate with temperature compensation and method for producing asurface wave component.

DE 199 09 692 C1 shows a flow measuring cell for studying a high-speedchemical reaction.

DE 101 04 957 A1 shows a method for producing a three-dimensional microflow cell.

The object of the invention is to make available a measuring cell whichhas improved performance characteristics, as well as a pertinentmeasuring system and a pertinent production method. In one embodiment,the measuring cell and the sensor and emitter are designed to be usedeven for high operating pressures, and for this purpose they are toexhibit high operational reliability.

This object is achieved by the measuring cell defined in claim 1 as wellas the measuring system defined in the independent claim and theproduction method defined in the independent claim. Particularembodiments of the invention are defined in the dependent claims.

In one embodiment, the object is achieved by a measuring cell for theinfrared analysis of fluids, especially by a measuring cell with anallowable operating pressure of more than 20 bar and preferably morethan 50 bar, with a flow channel for the fluid which is formed between afirst transparent element and a second transparent element, each beingtransparent at least in sections to infrared radiation, and the infraredradiation can be irradiated into the flow channel via the first elementand can exit from the flow channel via the second element, and the twoelements are connected fluid-tight to one another with high mechanicalstrength by a connecting layer of glass-containing material, especiallyof a sintered glass-ceramic material, which layer is located between thetwo elements.

Here it is advantageous that even comparatively thick elements, as arenecessary for the high pressure use, can be permanently and reliablyconnected to one another by the connection layer, especially that theconnection can be produced without porous spots in spite of thestiffness of the elements which are comparatively thick with respect tohigh pressure use. In the still unsintered state, by applying acorresponding pressure, the material of the connecting layer can bebrought into contact with the surfaces of the two elements such that atopography or ripple of the surfaces of the two elements which may bepresent is equalized in this way. This is especially advantageous whenthe measuring cells are fabricated in a panel; i.e., boards or wafers onwhich a plurality of elements and thus a plurality of measuring cellsare implemented at the same time are used, for example, for thecomponents.

For example, the elements in a panel can be formed from a silicon waferwith a thickness of more than 1 mm, especially more than 1.5 mm andpreferably more than 2 mm, and the connecting layer in the sinteredstate has a thickness of more than 50 μm and less than 500 μm,especially more than 100 μm and less than 300 μm, and preferably morethan 120 μm and less than 200 μm. The flow channel for the fluid can bea microfluid channel with a length of more than 3 mm, especially morethan 6 mm and preferably more than 9 mm, and with a width of less than10 mm, especially less than 8 mm and preferably less than 6 mm. In theflow channel, there can be one or more spacers by which even under theeffect of high pressure the height of the flow channel is kept to adefinable value. The spacers can be formed, for example, by webs whichrun lengthwise to the flow direction. The spacers and/or the geometry ofthe flow channel can be formed at least in sections by one of theelements and/or by the connecting layer.

In one embodiment, the connecting layer is applied structured to one ofthe elements or is placed between the two elements. By structuring theconnecting layer, for example, the flow channel can be defined, andparticularly the two elements bordering the flow channel can also befundamentally unstructured. Alternatively or in addition, the twoelements can also have, at least in sections, a structure which definesthe flow channel and which is produced by etching onto the surface.Fundamentally, the connecting layer can also be applied by all methodswhich are known, for example, from thick film technology.

In one embodiment, the connecting layer in the form of a strip, a tape,or a membrane is laminated onto one of the elements or is laminatedbetween the two elements. For example, the connecting layer in membraneform can be placed on a wafer which forms the first elements of aplurality of measuring cells, and a wafer which forms the secondelements of the plurality of measuring cells can be placed on theconnecting layer, and then the combination can be pressed together andthen sintered.

In one embodiment, the connecting layer has exit channels for the exitof organic components from the connecting layer in a process whichprecedes the sintering. The exit channels can be formed by alattice-like structure of the connecting layer. Providing these exitchannels is especially advantageous in the production of the measuringcells in a panel, because in this case the organic components which arevolatile in temperature treatment can emerge laterally.

In one embodiment, the connecting layer is formed from a low-temperaturecofired ceramic which preferably has plasticizers. A lamination of theconnecting layer by the plasticizers is possible. In the not yetsintered state, the connecting layer is flexible. Components of theconnecting layer in this state can be glass, especially borosilicateglass, borofloat glass, and/or quartz glass, ceramic—for exampleAl2O3—and organic components which volatilize during setting. The mixingof these components ensures the matching of the coefficient of thermalexpansion in the temperature range from −50 to +850° C. to thecoefficient of thermal expansion of the elements of the measuring cell,especially to the coefficient of thermal expansion of silicon.

In one embodiment, the connecting layer in a temperature range between 0and 200° C., especially between 0 and 400° C. and preferably between 0and 600° C., has a coefficient of linear thermal expansion whichdeviates less than 8 ppm/K, especially less than 5 ppm/K and preferablyless than 0.5 ppm/K from the coefficient of linear thermal expansion ofat least one of the elements, preferably of the two elements. In thisway, good matching of the coefficient of thermal expansion from theconnecting layer to the element is ensured so that the thermally inducedstresses are low even in the sintered state of the measuring cell, andthus a high operational reliability is guaranteed.

In one embodiment, at least one of the two elements, on one surfaceforming the boundary for the flow channel, has a surface structure whichacts as an antireflection layer and/or filter layer for the infraredradiation and/or as adhesion promoter for the connecting layer. Thetransmission capacity of the measuring cell for infrared radiation canthus be significantly increased, as a result of which a high signallevel arises for the evaluation of the sensor signal. Furthermore, inthis way, an optical filter can also be integrated into the measuringcell, by means of which filter the absorption bands of the fluid to bestudied can be determined. Moreover, in this way, the adhesive force ofthe connecting layer can be increased; this is especially advantageousin high pressure operation. The surface structure can be formed by ananostructure on the surface.

In one embodiment, the surface structure has a plurality of needles witha density of more than 10,000 needles per mm², especially more than100,000 needles per mm², and preferably more than 500,000 needles permm². Such needle-shaped elements can be produced, for example, insingle-crystalline silicon by self-masked dry etching. The surfacestructure which has been produced in this way according to its opticalappearance is also referred to as “black silicon.”

In one embodiment, the needles have a length of more than 0.3 and lessthan 30 μm, especially more than 0.5 and less than 15 μm, and preferablymore than 0.8 and less than 8 μm. Studies have shown that at this needlelength, an especially favorable antireflection behavior for infraredradiation and/or a high adhesion to the connecting layer can beachieved.

In one embodiment, the element has the surface structure also in theregion of the connecting layer. Here it is advantageous that the surfacestructure, alternatively or in addition to its action as antireflectionlayer, is also used as an adhesion promoter for the connection of theelement and the connecting layer. In particular, the needles canpenetrate into the structure of the connecting layer, and a large-areaconnecting layer is thus formed by the high surface: volume ratio of theneedles.

In one exemplary embodiment, the two elements are formed fromsingle-crystalline silicon. It has a relatively high coefficient oftransmission for infrared radiation and moreover excellent mechanicalproperties. Moreover, the elements of single-crystalline silicon can bestructured in almost any way with high precision in order to define flowchannels, using known structuring methods from semiconductor technology,including dry chemical and wet chemical etching methods.

In one embodiment, at least one of the two elements has a thickness ofmore than 1 mm, especially more than 1.5 mm and preferably more than 2mm. With such thick elements, especially in conjunction with thematerial single-crystalline silicon, measuring cells with highmechanical strength, which are thus also suitable for high pressure use,can be produced. The height of the flow channel which is defined by thethickness of the connecting layer can be between 50 and 500 μm,especially more than 80 μm and less than 400 μm, and preferably morethan 100 μm and less than 300 μm.

The invention also relates to the structure of a measuring system forthe infrared analysis of fluids with a measuring cell as describedabove, as well as an emitter and a sensor. The measuring system has anemitter for the infrared radiation, for example, a broadband-emittingheat radiator and/or a comparatively narrowband-emitting infrared lightemitting diode, and a receiver for the infrared radiation. Emitters andreceivers are preferably located on opposite sides of the measuringcell. In one unit, the receiver can have several detector elements bymeans of which the intensity of the radiation in different wavelengthranges can be measured. For this purpose, the receiver can have severalentry windows via which radiation is incident on one of the detectorelements. The windows and/or the detector elements can enable filtering.

Likewise, there can also be several emitters with a narrowband emission.

In one embodiment, the measuring system has an installation element witha receiving opening for the measuring cell. The measuring cell can beinserted into the receiving opening, in particular, the receivingopening can be adjusted with respect to its contour at least in sectionsto the outer contour of the measuring cell which can be, for example,polygonal and/or especially rectangular. The installation element hasone entry opening and one exit opening for the fluid. The fluid canenter the flow channel of the measuring cell via the entry opening, andthe fluid can emerge from the flow channel of the measuring cell via theexit opening.

The invention also relates to a method for producing a measuring cell,as described above. The connecting layer of glass-containing, especiallyglass-ceramic material, can be located between the two elements in thenot yet sintered state, for example, in the form of a strip or amembrane. The connecting layer here is a green compact. The connectinglayer can have calibration markings or openings by means of which theconnecting layer can be calibrated to the carrier of the elements. Theconnecting layer can be present in the form of an unsintered foil and/orcan be made from a mixture of borosilicate glass, quartz glass, andaluminum oxide as well as organic solvents.

The connecting layer is laminated as a green compact, for example, witha thickness of 300 μm under a pressure of 250 bar and at a temperatureof 70° C., between the two wafers forming the elements.

The connecting layer under this loading flows through the plasticizerswhich have been introduced in the green compact and equalizes allspacing tolerances between the two elements so that the connecting layeris in contact with the elements over the entire wafer surface.

The elements on their surface facing the connecting layer arenanostructured, for example, with the formation of needles. The needlespenetrate into the structure of the connecting layer. Then the sinteringprocess takes place under the action of pressure and temperature. At atemperature starting from approximately 650° C., the glass frits areconnected both to all components of the ceramic green compact and alsoto the needles of the wafers forming the elements. These needles arepresent in a nanostructure since in particular their lateral dimensionsare very small. The application of pressure in the sintering processessentially prevents a lateral shrinkage of the connecting layer. Theshrinkage of the connecting layer perpendicular to the surface of thewafers which form the elements can be about 50%.

Other advantages, features, and details of the invention will becomeapparent from the dependent claims and the following description inwhich several exemplary embodiments are described in detail withreferences to the drawings. The features referred to in the claims andin the specification may be critical for the invention individually orin any combination.

FIG. 1 shows a perspective view of one exemplary embodiment of ameasuring cell according to the invention,

FIG. 2 shows a section through a measuring system for the infraredanalysis of fluids,

FIG. 3 shows a perspective view of the installation element,

FIGS. 4 to 7 show the transmission behavior of a total of five fluidsamples at four different wavelengths,

FIGS. 8 to 10 show different stages of the method for producing ameasuring cell.

FIG. 1 shows a perspective view of one exemplary embodiment of ameasuring cell 1 according to the invention for the infrared analysis offluids for high pressure operation. The flow channel 10 for the fluid inthe exemplary embodiment has a width 12 of 5 mm, a length 14 of 9.5, anda height 16 of 0.2 mm. The flow direction is indicated by the arrow 18.In the flow direction 18 in the flow channel 10 in the middle withrespect to the width 12, a spacer 20 extends whose length 22 is about50% of the length 14 of the flow channel 10, in the exemplary embodimentapproximately 4.5 mm, and whose width 24 is less than 20% of the width12 of the flow channel 10, in the exemplary embodiment 0.8 mm. On theone hand, the height 16 is also stabilized in the middle region of theflow channel 10 by the spacer 20 and, moreover, the spacer 20, which ismade web-shaped, can also be used to improve the laminar flow in theflow channel 10.

The flow channel 10 is formed between a first element 2 and a secondelement 4; the two elements are transparent to infrared radiation atleast in sections and can consist of single-crystalline silicon Infraredradiation can be irradiated into the flow channel 10 via the firstelement 2, and the infrared radiation can emerge from the flow channelvia the second element 4. The two elements 2, 4 are connected to oneanother fluid-tight with high mechanical strength by a connecting layer6 located in between, consisting of a glass-containing material,especially of a sintered glass ceramic material.

FIG. 2 shows a section through a measuring system 8 for the infraredanalysis of fluids with a measuring cell 1 as described above. Themeasuring cell 1 is arranged in a system housing 28 by means of aninstallation element 26. FIG. 3 shows a perspective view of theinstallation element 26 which has a receiving opening 30 into which themeasuring cell 1 can be inserted. The receiving opening 30 isessentially matched to the outside contour of the measuring cell 1,which in turn is essentially rectangular, or in the special case,square. On its corners, the receiving opening 30 has bulges whichfacilitate the insertion of the measuring cell 1. The installationelement 26 has an entry opening 32 and an exit opening 34 via which thefluid can enter the flow channel 10 of the measuring cell 1 or canemerge from the flow channel 10 of the measuring cell 1. The connectionbetween the installation element 26 and the measuring cell 1 isfluid-tight, the sealing means which may be necessary for this purposesuch as, for example, gaskets or the like not being shown in FIG. 2 forreasons of clarity.

The measuring system 8, on one side assigned to the first element 2 ofthe measuring cell 1, has an emitter 36 for infrared radiation. Theemitter 36 can be, for example, a comparatively broadband-emittingheating element which in any case has a sufficient radiation intensityin the wavelength range of interest, for example, between 2 and 6 μm.The emitter 36 is preferably detachably fixed on the system housing 28by means of a fastening element 40 which has a central passage opening38. The emitter 36 radiates essentially centrally onto the first element2 of the measuring cell 1.

On the side opposite the measuring cell 1, in the measuring system 8,there is a receiver 42 which is located opposite the outer surface ofthe second element 4 and preferably centrally with reference to thesecond element 4 and thus to the measuring cell 1. In the illustratedexemplary embodiment, the receiver 42 has a total of four detectorelements 44, 46, of which FIG. 2 shows only two detector elements 44,46, as a result of the sectional view. On its surface facing themeasuring cell 1, the receiver 42 has a total of four windows 48, 50which are assigned to one detector element 44, 46 at a time. Each of thewindows 48, 50 and/or each of the detector elements 44, 46 can have afilter layer so that only one narrowband region at a time of theinfrared spectrum which has passed through the measuring cell 1 andwhich has been radiated from the emitter 36, is emitted onto thedetector element 44, 46.

FIGS. 4 to 7 show the relative transmission behavior T_(r) of a total offive fluid samples which are different with respect to their qualitystate at different wavelengths between 2.58 μm and 3.01 μm. Sample No. 1is a fresh and still unused fluid, the aging increasing with the samplenumber. As FIGS. 4 and 5 show, at the wavelengths 2.58 and 2.73 μm, noaging-dependent absorptions of the fluid can be measured. Conversely,according to FIGS. 6 and 7, at the wavelengths 2.87 and 3.01 μm,aging-dependent absorptions occur, the wavelengths at which theseabsorption bands occur allowing conclusions regarding the aging-dictatedcomponents in the fluid. The circumstance that at certain wavelengths noaging-dictated absorption can be measured (FIGS. 4 and 5) makes itpossible to use these wavelengths as reference bands in order, forexample, to take a comparison measurement.

FIGS. 8 to 10 show different stages of the method for producing ameasuring cell 1, as described above. For reasons of betterrepresentation, the dimensions are not shown to scale. First, ananostructure 56, for example, a surface structure of a plurality ofneedles, is applied to the silicon wafers 52, 54, which form the firstelements 2 and the second elements 4 on at least one surface, blanketedor structured. This intermediate stage is shown in FIG. 8.

FIG. 9 shows how the connecting layer 6 with a thickness ofapproximately 300 μm is laminated between the two silicon wafers 52, 54,preferably by the action of pressure between 50 and 500 bar, especiallybetween 200 and 300 bar, and preferably 250 bar, and temperature between50 and 100°, especially between 60 and 80° and preferably 70°. Theconnecting layer 6 can consist of a glass-containing and especiallyglass-ceramic material, for example, of a so-called LTCC ceramic. Theconnecting layer 6 can be laminated in the form of a tape as a greencompact. The connecting layer 6 flows under the lamination pressurethrough the plasticizers placed in the tape and thus equalizes allspacing tolerances between the silicon wafers 52, 54. Here the needlestructure 56 penetrates into the surface of the connecting layer 6.

The structuring of the tape and/or of the silicon wafers ensures theremoval of organics from the tape in the debinding process through addedchannels. The flatness defects of the elements 2, 4, especially in theproduction in a panel, are mitigated by the connecting layer 6,especially by its properties prior to the sintering process.

FIG. 10 shows the state after sintering which takes place at atemperature of more than 600°, preferably more than 750° and, forexample, between 800 and 900°. Here the glass components of theconnecting layer 6 combine with all components of the ceramic-containingtape as well as with the nanostructure 56 of the silicon wafers 52, 54.The application of pressure during the sintering process essentially oreven completely stops lateral shrinkage of the glass ceramic. Theshrinkage perpendicular to the surface of the silicon wafers 52, 54 isabout 50% so that in the end the connecting layer 6 is present in athickness of approximately 150 μm.

1. A measuring cell (1) for the infrared analysis of fluids, especiallya measuring cell (1) with an allowable operating pressure of more than20 bar and preferably more than 50 bar, with a flow channel (10) for thefluid, which is formed between a first transparent element and a secondtransparent element (2, 4), each of which is transparent at least insections to infrared radiation, and the infrared radiation can beradiated into the flow channel (10) via the first element (2) and canexit from the flow channel (10) via the second element (4), and the twoelements (2, 4) are connected fluid-tight to one another with highmechanical strength by a connecting layer (6) of glass-containingmaterial, especially a sintered glass-ceramic material, which layer islocated between the two elements.
 2. The measuring cell (1) according toclaim 1, characterized in that the connecting layer (6) is appliedstructured to one of the elements (2, 4).
 3. The measuring cell (1)according to claim 1, characterized in that the connecting layer (6) inthe unsintered state is laminated onto one of the elements (2, 4) or islaminated between the two elements (2, 4).
 4. The measuring cell (1)according to claim 1, characterized in that the connecting layer (6) hasexit channels for the exit of organic components from the connectinglayer (6) in a process which precedes the sintering.
 5. The measuringcell (1) according to claim 1, characterized in that the connectinglayer (6) is formed from a low-temperature cofired ceramic which hasplasticizers.
 6. The measuring cell (1) according to claim 1,characterized in that the connecting layer (6) is structured as aceramic film and in this way the flow channel (10) is defined in itsshape.
 7. The measuring cell (1) according to claim 1, characterized inthat the sintered connecting layer (6) in a temperature range between 0and 200° C., especially between 0 and 400° C. and preferably between 0and 600° C., has a coefficient of linear thermal expansion which hasless than 8 ppm/K, especially less than 5 ppm/K and preferably more than3 ppm/K.
 8. The measuring cell (1) according to claim 1, characterizedin that at least one of the two elements (2, 4) on one surface formingthe boundary for the flow channel (10) has a surface structure (56)which acts as an antireflection layer and/or filter layer for theinfrared radiation and/or as adhesion promoter for the connecting layer(6).
 9. The measuring cell (1) according to claim 8, characterized inthat the surface structure (56) has a plurality of microneedles with adensity of more than 10,000 needles per mm2, especially more than100,000 needles per mm2, and preferably more than 500,000 needles permm2.
 10. The measuring cell (1) according to claim 9, characterized inthat the needles have a length of more than 0.3 and less than 30 μM,especially more than 0.5 and less than 15 μM, and preferably more than0.8 and less than 8 μm.
 11. The measuring cell (1) according to claim 8,characterized in that the element has the surface structure (56) also inthe region of the connecting layer (6), and that the surface structure(56) also forms an adhesion promoter for the connection of element (2,4) and connecting layer (6).
 12. The measuring cell (1) according toclaim 1, characterized in that the two elements (2, 4) are formed fromsingle-crystalline silicon.
 13. The measuring cell (1) according toclaim 1, characterized in that at least one of the two elements (2, 4)has a thickness of more than 1 mm, especially more than 1.5 mm andpreferably more than 2 mm.
 14. The measuring cell (1) according to claim1, characterized in that microfluidic structures are fabricated in atleast one of the two elements (2, 4), preferably in both elements (2,4), and that the microfluidic structures are fabricated preferably inone silicon wafer which forms the elements (2, 4).
 15. A measuringsystem (8) for the infrared analysis of fluids with a measuring cell (1)according to claim 1 and with an emitter (36) for the infrared radiationand a receiver (42) for the infrared radiation.
 16. The measuring system(8) according to claim 15, characterized in that the measuring system(8) has an installation element (26) with a receiving opening (30) intowhich the measuring cell (1) is inserted, and wherein the installationelement (26) has an entry opening (32) and an exit opening (34) for thefluid via which fluid can enter the flow channel (10) of the measuringcell (1) and can emerge from the flow channel (10) of the measuring cell(1).
 17. A method for producing a measuring cell (1) according to claim1, characterized in that a connecting layer (6) of a glass-ceramicmaterial in the not yet sintered state is located between the twoelements (2, 4), and then the arrangement of the two elements (2, 4) andthe connecting layer (6) are sintered by temperature and action ofpressure, and in doing so a fluid-tight connection with high mechanicalstrength is produced.